Preheat oven to 350 degrees. Spread 2 tablespoons peanut butter on each flatbread. Evenly disperse sausage over peanut butter. Sprinkle with red pepper flakes if desired. Divide celery and apples evenly over sausage, sprinkle cheese over celery and apples. Bake on a cookie sheet sprayed with nonstick cooking spray for a softer crust, or directly on the oven rack for a crisper crust until the cheese is melted (about 8 minutes). Sprinkle with the basil and cut into quarters to serve.